I have a new GMS 1.20 two-stroke with a Bisson Pitts style muffler installed in a new Extra 330. The engine was broken in with 3/4 gallon of Wildcat Premium Extra 10%Nitro and 18% castor/synthetic blend and APC 16x6. I installed a hotter than normal Enya #3 plug to minimize an overly rich mid-range ( a common thread subject ) and was very happy with the engine and its incredibly low idle. On the fifth flight I started the engine and began to lean the top end using my tach( I've been running it 400-500 RPM below peak during the first few flights-it has a "Dykes" piston ring) when it suddenly quit and nearly seized during the run-up. I went to restart and noticed the engine was VERY tight - similar to an engine that has sat for years with congealed fuel in it. I put some after-run oil in it and turned it over and over until it loosened up enough to restart, which it did with no apparent power loss. After another uneventful flight, I checked it and it was still VERY tight, although it it is equally tight throughout the turn with absolutely no grinding. I don't want to pull a new engine out, but this thing is much tighter than when new and much tighter than any of my 12 other assorted ringed,ABC, ABN, etc.
Any ideas?

I had an engine do that once....it was because the can of fuel did not have
enough oil in it. Can you dump a few ozs. of oil in it for another try ??

Dave.

When the engine is tight, is it tight right throughout the revolution, without loosening up at TDC or BDC. In other words, is it the bearings that are tightening or the piston/liner assy?

I recently saw a brand new OS .46 AX do exactly this - run perfectly and as it warmed up, start tightening up until it stopped. It would gradually loosen up as the engine returned to room temperature. The main bearings themselves were tightening up on this engine - we theorised that one of the bearings may not have been pressed home in the case properly. We removed the prop and thrust washer (the AX uses a flat rather than a taper - I'm not sure what the GMS 1.20 has) and checked for endfloat and reassembled carefully and the problem was solved!

It might be worth a try.

Good luck

Thanks,
The engine is equally tight throughout its full turn with absolutely no noticeably tight spots and no grinding - my first thought was a seized/spun bearing, but the engine still seems to run fine even with this problem.